1. Field of the Invention
The present invention relates generally to combustion detection and, more particularly, to a system for detecting combustion through the use of ionization current sensing.
2. Description of the Art
Two challenges facing vehicle manufactures are the improvement of fuel economy and the reduction of emissions without significantly increasing vehicle costs. One way of accomplishing such tasks is through the use of detecting various combustion states in the cylinders of internal combustion engines. Such feedback information can then be supplied to various engine components which can be adjusted to action in accordance with the combustion cycle. Detection of such combustion may be obtained by sensing ionization current which may occur given the existence of ions in a combustion cylinder. The ions are typically generated within the cylinders due to chemical reactions between compressed air and fuel subject to combustion. The concentration of ions, indicative of the combustion state in any given cylinder, can be detected by the means of ionization current flowing across air gaps which exist in the cylinder. Applying a voltage across an air gap in a cylinder, when ions are present, allows for the detection of a vehicle combustion state by sensing ionization current which flows across the gap. Typically, the air gap between the electrodes of a spark plug which is already disposed within each cylinder, is used to take measurements for determining the amount of ionization current flow.
Prior current sensing ionization circuits, consisting of distributor, distributor-less, and "coil-on-plug" ignition systems, require the use of high voltage diodes to protect the ionization circuit from voltage spikes during combustion. Such circuits are costly given the high expense of the high voltage diodes. Furthermore, since the diodes are required to be connected between each cylinder and the ionization circuit, if one of the diodes is shorted, there will be no ignition in the corresponding cylinder. Prior ionization circuits also do not deal effectively with "smoothing out" the excited ionic current wave form which is produced on combustion. Such circuits further require the use of an extra signal processing unit to process the ionic current wave form. The use of such a signal processor is costly and can increase the size and number of components needed to detect combustion via ionization current sensing. The additional processing unit may also produce electromagnetic interference with other circuit componentry.
Therefore, it would be desirable in the art of motor vehicle to have an ionization current sensing circuit that is inexpensive to produce, has relatively few components, and produce a smooth current ionization output signal.